Recently, electronic devices are evolving towards miniaturization, lower cost and higher performance. In order to meet this requirement, passive components, which occupy the largest area in circuit design, need to be improved. In the traditional Surface Mounted Technology (SMT), most of passive components used in electronic devices, such as capacitors and inductors, are discrete components that take up a large real estate of a Printed Circuit Board (PCB). Meanwhile, their performances and reliabilities are not good due to large parasitic inductances caused by long interconnections between components and solder joints.
The System-On-Package (SOP) concept where components are integrated as part of a package housing an electronic device is an attractive option to build wireless communication modules. This concept provides a potential for higher integration, leading to a more compact module size that is critical for portable applications. In addition, SOP components have reduced cost since they eliminate the need for discrete components and have shortened assembly time. Also, their performances are superior to those of on-chip components.
Recent progresses in commercial communications have created a need for RF components having more compact sizes. Lumped elements can provide circuit miniaturization at low frequency while having spurious-free responses at high frequency. Therefore, it is of particular interest to implement quadrature hybrids with lumped elements. The Low Temperature Co-fired Ceramic (LTCC) technique is a technique for integrating passive components. However, for SOP applications, the LTCC technique has two major disadvantages: it has a high fabrication cost and it is incapable of dealing with high density interconnections. On the other hand, the low cost organic substrate technique has been well developed, whereby high-density interconnections are possible. Further, embedded passive technique has been proposed and become an attractive technology with benefits of lower parasitic parameter, better performance, compactness and higher reliability. The embedded passive technique based on organic substrate has been widely studied because of its advanced characteristics. It provides a cost-effective alternative to discrete passive components and LTCC devices.
Quadrature hybrid, also known as branch-line coupler, is an elemental component in various microwave circuits, such as balanced amplifiers, balanced mixers, phase shifters and beam-forming networks for array antennas. Conventional distributed-element quadrature hybrids, such as branch-line or Lange couplers, typically occupy large areas in Microwave Integrated Circuits (MICs) and it's impossible to implement a planar form quadrature hybrid in a limited circuit area, especially at low frequency. Semi-lumped and lumped elements can be used to construct quadrature hybrids for circuit size reduction. Semi-lumped and lumped element quadrature hybrids are constructed from lumped π or T network and/or lumped distributed networks equivalent to a transmission line section with an appropriate characteristic impedance and electrical length. However, conventional semi-lumped and lumped element quadrature hybrids can only achieve a fractional bandwidth of 1%˜2% and are thus not suitable for wideband applications.
Reference [1], J. Yamasaki, I. Ohta, T. Kawai and Y. Kokubo, “Design of broadband semi-lumped and lumped element quadrature hybrids,” 2005 IEEE MTT-S Int. Microwave Symp. Dig., vol. 3, pp. 1247-1250, June, 2005, discloses semi-lumped and lumped element quadrature hybrids. However, in this solution, lumped elements (e.g. capacitors and inductors) are located on the surface of a PCB, which is not desirable for size reduction. Semi-lumped elements (e.g., microstrip lines) also occupy a large area, especially at low frequency. Performances of the lumped elements on the PCB surface are vulnerable to environmental conditions, such as temperature, humidity and electromagnetic interferences. In addition, the Q-factors, or Q-values, of the equivalent capacitors and inductors are limited due to their planar structures. There are many parameters to be optimized in circuit design due to the complicated topology. Moreover, these quadrature hybrids can only achieve a fractional bandwidth of 25%.
Reference [2], J. Hou and Y. Wang, “A compact quadrature hybrid based on high-pass and low-pass lumped elements,” IEEE Microw. Wireless Compon. Lett, vol. 17, no. 8, pp. 595-597, August, 2007, discloses a quadrature hybrid with a semi-planar structure. However, it cannot be used at low frequency (e.g., lower than 200 MHz) due to limited capacitance and inductance. The performances of its components are also vulnerable to environmental conditions (e.g., temperature, humidity). In addition, the Q-values of the equivalent capacitors and inductors are limited. Moreover, this quadrature hybrid can only achieve a fractional bandwidth of 18% while requiring relatively large capacitance and inductance, which is undesirable for size reduction.
Reference [3], Y. J. Lee and J. Y. Park, “Fully embedded lumped LC-quadrature hybrid coupler into organic packaging substrate for power sampling,” Microw. Optical Technology Letters, vol. 51, no. 3, pp. 845-848, March, 2009, discloses a lumped LC-quadrature hybrid. It uses planar inductors and capacitors to shorten the length of the branch. However, it cannot be used at low frequency (e.g., lower than 200 MHz) either due to limited capacitance and inductance. In addition, the Q-values of the planar inductors are very low, which results in a high insertion loss of passive components. There are many parameters to be optimized in circuit design due to the complicated topology. Moreover, it can only achieve a fractional bandwidth of 10%.
Therefore, there is a need for an improved quadrature hybrid capable of overcoming at least some of the above problems in the prior art.